I'm getting great flashbacks to the 1980s, and the DFC "green army man" style Forest of Doom playset... the forest was little more than a plastic play-mat, with a couple large sheets of 2-D cardboard trees and tree-monsters, but that cardboard scenery was a thing of beauty, really helping to sell the playset's fantasy setting, such as it was - the folks responsible for the art direction and production on this playset and the others in the series really deserve more credit than they got:
I think we've gotten kind of spoiled by the high quality of modern miniatures, but a little printed cardboard went a long way back then on practically no budget at all, and there's no reason an evocative modern 28mm haunted forest should cost a hundred bucks for three trees!
I think those toy trees in Grumpy Gnome's last post above are a nice hint at what can be done with inexpensive "green army man" style plastic, and relatively simple manufacturing techniques: those toy trees really aren't very detailed, elaborate, or natural-looking, but they more than get the point across, they surely wouldn't cost a lot on their own if only they were sold on their own as a set, and I bet they paint up really nicely into a moody haunted forest.
Something similar could easily be done for man-eating tree-demons, too, for that matter! (Larger tree-demon figures need not be solid plastic, either - a two-part front-and-back hollow body and arms with branches on the top for those leafy bits might work nicely...)
Not sure that Wargames Atlantic's hard plastic manufacturing is the best way to produce that sort of project, but surely someone out there can find a way to do some Made-in-the-USA soft plastic terrain pieces and even monsters like that, for something comparable to a dollar-store green-army-man budget!
Short of that, I think those toy trees, the small trees that come in some 1/72 scale battlefield-accessory scale model sets, and the Mantic Terrain Crate trees are decent enough (if a bit small) for a haunted forest, and that Reaper Tree of Woe makes a fine centerpiece.
CMON also made a nice batch of plastic trees, somewhat similar to Mantic's, for their "HATE" miniature war game, but I'm not sure how easy it is to find those, and they're likely to be kinda pricey.
It's maybe also worth a reminder that there is also a small but prolific industry out there dedicated to making strinkingly realistic trees for model railroading - some assembly required - which can serve some haunted forest projects, with a little creativity.
But, I think I've still got a soft spot in my heart for the old '80s DFC cardboard cut-out forest: in a hobby where things can sometimes tend toward over-engineering, over-complication, and over-expensiveness, that cardboard scenery is a reminder that there doesn't have to be one way to do this! :)